I. Field of the Invention
The present invention relates generally to motorized riding trowels for finishing concrete. More particularly, the present invention relates to transmissions for powered riding trowels of the type classified in United States Patent Class 404, Subclass 112, and particularly to CVT transmission systems for such trowels.
II. Description of the Prior Art
It has long been recognized by those skilled in the art that freshly placed concrete must be appropriately finished. Proper and timely finishing insures that desired surface characteristics including appropriate smoothness and flatness are achieved. Motorized riding trowels are ideal for finishing very large areas of plastic concrete quickly and efficiently, and such trowels have become a standard in the industry.
A typical power riding trowel comprises two or more bladed rotors that project downwardly and frictionally contact the concrete surface for finishing. These rotors are driven by one or more motors mounted on the frame. Typically the motors drive suitable reduction gearboxes (i.e., 20:1 reduction) to power the rotors. The riding trowel operator sits on top of the frame and controls trowel movement with a steering system that tilts the axis of rotation of the rotors. The weight of the trowel and the operator is transmitted frictionally to the concrete by the revolving blades. The unbalanced frictional forces caused by rotor tilting enable the trowel to be steered.
Holz, in U.S. Pat. No. 4,046,484 shows a pioneer, twin rotor, self propelled riding trowel. U.S. Pat. No. 3,936,212, also issued to Holz, shows a three rotor riding trowel powered by a single motor. Although the designs depicted in the latter two Holz patents were pioneers in the riding trowel arts, the devices were difficult to steer and control.
Prior U.S. Pat. No. 5,108,220 owned by Allen Engineering Corporation, the same assignee as in this case, relates to an improved, fast steering system for riding trowels. Its steering system enhances riding trowel maneuverability and control. The latter fast steering riding trowel is also the subject of U.S. Pat. No. Des. 323,510 owned by Allen Engineering Corporation.
Allen Engineering Corporation U.S. Pat. No. 5,613,801 issued Mar. 25, 1997 discloses a power riding trowel equipped with twin motors. The latter design employs a separate motor to power each rotor. Steering is accomplished with structure similar to that depicted in U.S. Pat. No. 5,108,220 previously discussed.
Allen U.S. Pat. No. 5,480,257 depicts a twin engine powered riding trowel whose guard structure is equipped with an obstruction clearance system. When troweling areas characterized by projecting hazards such as pipes or ducts, or when it is necessary to trowel hard-to-reach areas adjacent walls or the like, the guard clearance structure may be retracted to apply the blades closer to the target region.
Allen U.S. Pat. No. 5,685,667 depicts a twin engine riding trowel using “contra rotation.” For enhanced stability and steering, the rotors rotate in a direction opposite from that normally expected in the art.
As freshly poured concrete “sets,” it soon becomes hard enough to support the weight of the specialized finishing trowel, so pan finishing can begin. By starting panning while concrete is still “green,” within one to several hours after pouring depending upon the concrete mixture involved, “super-flat” and “super-smooth” floors can be achieved. The advent of more stringent concrete surface finish specifications using “F” numbers to specify flatness (ff) and levelness (fl), dictates the use of pans on a widespread basis.
The panning process comprises three different recognizable stages. In the initial “brake open” stage, the rotors are ideally driven between 40 and 65 RPM. As the concrete hardens, the pan floating stage occurs, involving rotor speeds between 70 and 95 RPM. The last phase of pan floating, the “fuzz stage,” uses an increased rotor speed of between 95-125 RPM. At present these RPM requirements are achieved simply by varying motor speed.
Pan finishing is normally followed by medium speed blade finishing, after the pans are removed from the rotors. An enhancement is the use of “combo blades” during the intermediate “fuzz stage” as the concrete continues to harden. So-called “combo-blades” are a compromise between pans and normal finishing blades. They present more surface area to the concrete than normal finishing blades, and attack at a less acute angle. The rotors are preferably turned between 100 to 135 RPM at this time. Finishing blades are then used, and they are rotated between 120 to 150 RPM. Finally, the pitch of the blades is changed to a relatively high contact angle, and burnishing begins. This final trowel finishing stage uses rotor speeds of between 135 and 165 RPM.
Modern large, high power riding trowels can deliver substantial horsepower. During use, however, the drive train, the gearboxes, the rotors and the motors are subject to substantial stresses. Motor loading varies as the rotor RPM requirements change. Furthermore, ideal rotation speeds can vary depending upon the concrete, whose frictional characteristics vary between the freshly poured and stricken off stage, the subsequent green stages, and the end stages occurring after final curing and hardening. The motors function most efficiently at a given operating point in their characteristic horsepower-RPM and torque-RPM curves. Especially with diesel engines, optimum torque and horsepower requirements are achieved over a limited RPM range.
The engines on most riding trowels directly power the reduction drive gear boxes connected to the rotor shafts. The incoming shaft speed of the conventional rotor gear box is the same as the motor output RPM. The gearbox output shaft speed (i.e., rotor speed) is reduced, approximately 20:1. Engine RPM is usually the key variable related to output power. However, with engine speed increases, excessive power may be developed and the finishing mechanism may rotate too fast. For example, the initial panning stage requires relatively high power because of the viscous character of the still-wet concrete, but relatively low rotor speeds are desired. Since the rotors are driven through a fixed ratio, established by the gearbox and pulleys, optimum engine power often cannot be obtained during panning without risking excessive rotor speeds.
It has thus proven desirable to provide a CVT riding trowel wherein the engine and gear boxes can operate at ideal speeds over a wide range of finishing conditions.
U.S. Pat. No. 5,967,696 Oct. 19, 1999 issued to Allen Engineering Corporation depicts a CVT riding trowel, i.e., a trowel with a variable ratio transmission. The trowel described in the latter patent includes a CVT drive train powering a pair of rotors. The rotors are shaft-driven by reduction gear boxes. The CVT system comprises a variable ratio pulley driven by the motor. A second variable ratio pulley drives the gear box input shaft, with a drive belt entrained between the twin, variable ratio pulleys. Means are provided to change the effective diameters of a pair of belt-coupled pulleys. The varying ratio between the pulleys establishes a variable, overall drive gear ratio. However, it has been found that with the latter design, the CVT pulleys do not operate at a high-enough speed to promote efficiency.
Other continuously variable transmission devices not specific to riding trowels are seen in U.S. Pat. Nos. 8,682,549, 8,668,607, 8,686,886, 7,063,633, 6,994,643, 7,081,057, 7,090,600, 6,569,043, 6,120,399, 6,958,025, 6,953,400, 6,155,940, and 5,377,774.
It has recently been realized that improved efficiency of the overall power train results where the CVT transmission system, in a riding trowel for example, can operate at what would otherwise be classified as an excessive speed. For example, the first stage couplings or pulleys in a conventional CVT system operate at the drive motor output shaft speed or RPM. Variable gear reduction offered to the gearbox drive shaft then reduces applied RPM from that of the motor. It has been discovered that a CVT system that first increases the RPM speed from the motor, to operate the CVT pulleys at higher-than-expected speed, results in efficiency gains. Subsequent gear reduction to the gearbox drive shaft enables the motor to run at its desired speed at maximum intervals, while facilitating proper gear box speed. At the same time, over-torque of the CVT pulleys is avoided and belt breakage is avoided.